I am relatively new to atom/html/css and am having trouble finding the elements I need to edit to change some syntax colors. The particular color in question is evident in the included image, specifically the green dotted underline that indicates a selected html tag pair. 
I have tried to find it with the developer tools but I haven’t managed to find anything matching the underline specifically.
The underline is actually not a normal text underline, but a separate rectangle drawn behind the text. You can find it with the css selector .bracket-matcher.region, and override the default border-bottom: 1px dotted lime with whatever you want in your stylesheet.